PROJECT SUMMARY/ABSTRACT With the lowest five-year survival rate of all major cancers, pancreatic adenocarcinoma (PDAC) currently has no effective treatment approach and is largely resistant to all chemotherapy agents. The prognosis for patients with PDAC is dismal with a 5-year survival rate of 7%. Clearly, development of novel therapies and preventive strategies for pancreatic adenocarcinoma (PDAC) is urgently needed. Instead of targeting the cancer cells alone, we propose an innovative strategy to block the functions of the much more abundant stromal fibroblasts in the tumor microenvironment which we have shown can promote PDAC tumor progression. We have identified Dickkopf-3 (DKK3) as a protein secreted by the stromal cells that stimulates tumor growth and metastasis as well as resistance to chemotherapy and radiation. DKK3 may also interact with the immune system and thereby inhibit the efficacy of immunotherapy drugs. We have developed novel blocking antibodies to DKK3 which in our preliminary studies, appear to be highly effective in inhibiting PDAC tumor growth and metastasis. Treatment with anti-DKK3 antibody alone was associated with improved survival in mice by 2-fold and the results were even more promising in a syngeneic immunocompetent model of PDAC. In addition, immunotherapy with checkpoint control inhibitors showed efficacy with improved survival in an autochthonous model of PDAC depleted of DKK3 whereas there was no effect when DKK3 was expressed. We hypothesize that PSCs in the microenvironment contribute to resistance to immunotherapy and that neutralization of DKK3 will reduce the immunosuppressive activity of PSCs. For our proposed studies, we will use novel DKK3 blocking antibodies developed in our lab and both orthotopic xenograft and autochthonous models of PDAC. Our specific aims are: (1) to evaluate the efficacy of combination therapy of DKK3 blockade with checkpoint control inhibition, and (2) to determine whether DKK3 expression is a predictive biomarker that correlates with response to either DKK3 targeted therapy or immunotherapy with checkpoint control inhibitors. DKK3 was expressed in nearly all the PDAC samples from patients that we examined and therefore, we expect that anti-DKK3 treatment has the potential to make a substantial impact on this deadly disease. Results of these studies will facilitate the development of innovative therapeutic strategies to target the tumor-associated stroma in PDAC and potentially overcome its resistance to immunotherapy.